A wide gamut of colors can be generated and displayed by intermixing red, green, and blue primary light sources in different ratios. In general, a color that matches any particular color sample can be produced on a display using the three primary colors.
The ratios of the red, green, and blue primaries that produce a color that matches any particular sample, however, depend on the ambient light used to illuminate the color sample. Moreover, especially when only the three primary colors are used, it is possible to produce a color that some people see as a match to a sample, but that other people see as a mismatch. Colors that look alike in some ambient illuminations look different in other illuminations, and colors that look alike to some people look different to others. These phenomena are referred to as “metamerism.”
Appearance of the color under given illumination depends on both the reflectance spectrum and the illuminance spectrum. Consequently, reproduction of the appearance of a color sample requires knowledge of both the complete reflectance spectrum and the complete illuminance spectrum. When these are known, the color can be reproduced by generating the spectrum that is the convolution of these two.
Perfect reproduction of a color requires control over the intensity level of each wavelength of the visible spectrum. In practice, the spectrum can be divided into a plurality of comparatively narrow bands, and the average intensity within each band can be specified. Thus, each band serves as a primary color. Using four or five such bands, or primary colors can significantly reduced metamerism, compared with the commonly used three primary colors. Using seven primaries reduces the metameric effects by an order of magnitude.
If the spectra selected for the different primary bands are narrow, then the primary colors will be intense and the color gamut that can be generated through a combination of the primaries will be large. Because a large color gamut is desirable, narrow-band primary colors are preferred.
The use of multiple narrow-band primary colors requires the use of multiple sources whose output intensity is accurately controlled and whose output is combined and displayed in a substantially uniform manner. A system for specifying, measuring, displaying, and communicating colors with reduced metameric confusion is described in U.S. Pat. No. 6,259,430 to Riddle et al., assigned to the assignee of this invention, and incorporated herein by reference. The '430 patent discloses several exemplary displays for using the invention, but there is still a need in the art for others that are particularly well suited to the practice of the invention. There is also a continued desire to develop novel and useful applications for the technology introduced by the '430 patent.
The problem of providing uniform illumination over a desired size target area using even a single light source is a vexing one. One known display 10 uses a set of integrating spheres in an arrangement such as shown in FIG. 1. A light source 11 in small integrating sphere 12 emits light which is sent into larger integrating sphere 16. Larger integrating sphere 16 has a display area 18, which is typically covered with a diffuser 22. A baffle 24 is typically used to shield the display area from the direct illumination of the source. One problem with this arrangement is that the source is located off to one side of the display area, which leads to asymmetries in the display because the light is introduced in an asymmetrical manner. This problem can be mitigated by making the sphere 12 larger compared to the size of the viewing window.
There is still a need for a display device, however, that will permit accurate reproduction of a light source with acceptable uniformity over a reasonable viewing area that approaches the diameter of the larger sphere. In particular, there is a need for displays and that will permit a user to effectively compare the color being displayed to the color of an actual sample in ways that are useful for applications where comparison with a precise color sample is desired.